This chapter investigates the impact and relative advantages of two engagement models for delivering science-specific PD: school-led partnerships and external-led PD delivered by STEM experts. Traditionally professional development (PD) led by professional education experts has been used to improve the quality of science teaching and learning in primary schools. Over almost 10 years in the UK, there has been a drive to school-led collaborative partnerships providing PD as part of an overall policy change to a school-led self-improving system. Using data collected from two exemplar programmes for subject-specific PD in primary schools, we examine the effectiveness, advantages and disadvantages of these two models of PD. Both programmes were successful in improving the leadership and teaching of science with some evidence of impact on pupil outcomes, yet they also demonstrated different advantages and challenges. The suggested solution for effective PD is to develop a 'blended' approach that incorporates the stronger elements of each of the models. [For the complete volume, "Bridging Research and Practice in Science Education: Selected Papers from the ESERA 2017 Conference. Contributions from Science Education Research. Volume 6," see ED615249.]